Dike control of hydrothermal circulation in the Tertiary Icelandic crust and implications for cooling of the seafloor

Hydrothermal activity along the Mid-Atlantic Ridge is predominantly high-temperature venting controlled by volcano-tectonic processes confined to the ridge axis and neotectonic zone, which extends ~ 20 km on each side of the axis (e.g. TAG or Logatchev 1). These vents cannot, however, account for all the heat which needs to be removed to cool the plate and a significant amount of heat is probably removed in the off-axis regions as well. These regions have previously not been systematically surveyed for hydrothermal activity due to a lack of predictive models for its nature, location or controlling structures. Here we use hot springs in the Tertiary Westfjords of Iceland as onshore analogs for hydrothermal activity along the off-axis Mid-Atlantic Ridge to better understand tectonic and volcanological controls on their occurrence, as well as the processes which support hydrothermal circulation. Our results show that even crust ≥ 10 Ma has abundant low-temperature hydrothermal activity. We show that 66% of hot springs investigated, and 100% of those for which a detailed geological setting could be determined, are associated with basaltic dikes cross-cutting the sub-horizontal lava sequence. This is in strong contrast to on-axis springs, which are known (both from underwater and on land) to be predominantly associated with faults. Absence of earthquakes in Westfjords suggests that the faults there are no longer active and possibly sealed by secondary minerals, suppressing fluid circulation. In such a situation, the jointed and fractures dike margins may provide the major pathways for fluid circulation. Extrapolating this idea to the off-axis regions of the Reykjanes Ridge, we suggest, based on bathymetric maps, potential sites for future exploration for off-axis hydrothermal systems.